Because polyvinyl chloride (PVC), which is inexpensive and has superior processability and is thus used as general-purpose plastics in various fields, is unstable to heat and light, hydrochloric acid is released from the molecular chain of PVC as soon as thermal decomposition begins in a forming process. In order to prevent hydrochloric acid from being released, a heat stabilizer is utilized. The heat stabilizer plays a role in capturing hydrochloric acid which continuously accelerates deteriorative reaction, thus stabilizing vinyl chloride.
The heat stabilizer which is mixed with resin is a compound for allowing the physical and chemical properties of resin to be retained during use of processed and finished products. Examples of the heat stabilizer of PVC include a lead stabilizer, an organotin stabilizer, a fatty acid metal salt stabilizer, etc.
The lead stabilizer has superior electrical insulating properties, weatherability and heat stability and is relatively inexpensive and is thus used in various fields including electrical wire coating, hard pipe, PVC tile, and PVC fitting, etc. However, the use of lead which is toxic is restricted from the environmental point of view. Further, because of a recent increase in the price of lead, the price advantage thereof is lost.
The organotin stabilizer has high transparency and heat stability and thus may be used alone without additional treatment. However, tin has no external activity and causes sulfur pollution problems by cadmium or lead or particular odors of sulfur compounds and therefore limitations are imposed on the use thereof from the environmental point of view, and further, tin is expensive. Examples of the organotin stabilizer include monodibutyl tin, monodioctyl tin, etc.
The fatty acid metal salt stabilizer is composed mainly of fatty acid metal salts including Ba, Ca, Cd, Pb and Zn as metal species. The metal species are not used alone, and may be used in combinations of two or more thereof, for example, Ba—Cd, Ba—Zn, and Ca—Zn. The Ba—Ca stabilizer is responsible for capturing hydrochloric acid and imparting heat stability owing to reproduction of a Zn salt through reaction with ZnCl2. The Ba—Cd stabilizer is a stabilizer having superior heat stability and high transparency, but the use thereof is restricted in many countries attributable to problems of cadmium that is a heavy metal. The Ba—Zn stabilizer has relatively good heat stability and transparency and is thus used for whole soft PVC products, and further, it is suitable for paste resin processing and has lower toxicity than the Ba—Cd stabilizer, thus facilitating a tendency to substitute for the Ba—Cd stabilizer. Because the Ca—Zn stabilizer is non-toxic, environmentally friendly and inexpensive, it is mainly used for food packaging materials, toys, food containers, and medical instruments but has inferior heat stability to other stabilizers. To solve this problem, the Ca—Zn stabilizer may be used together with an assistant stabilizer or in a large amount. However, the use of the large amount of Ca—Zn stabilizer may negatively affect the properties of products.
Hence, with the goal of solving the problems of the aforementioned heat stabilizers, thorough research has been conducted. In particular, hydrotalcite stabilizers are under intensive study.
Hydrotalcite, which is represented by [Mg6Al2(OH)16CO3.4H2O], functions as a heat stabilizer through ion exchange of CO32− ions, which are located between Mg—Al layers, with halogen ions.
Hydrotalcite is commercially synthesized in high purity and may be applied in various fields, including environmental pollutant scavengers, catalyst inactivators, acid scavengers, acid adsorbents, halogen adsorbents, flame retardants, assistant flame retardants, enhancers for increasing heat resistance stability of polymer, acid neutralizers, UV blocking agents, heat insulation agents, and chlorine-resistant agents.
Examples of the preparation of hydrotalcite include 1) a co-precipitation process using a water-soluble meal salt and 2) a hydrothermal synthesis process at high temperature under high pressure using poorly soluble metal oxide. Prior techniques thereto include Korean Patent Registration No. 10-454273 which discloses a method of preparing hydrotalcite using polyhydric alcohol and polyhydric alcohol ester or a metallic substituent, and Korean Patent Registration No. 10-601016 which discloses a method of preparing nano-sized hydrotalcite which is Mg—Al layered double hydroxide. Also, Korean Patent Registration No. 10-775602 discloses a method of preparing hydrotalcite having a uniform particle size, including preparing an admixture of water-soluble metal salt, an alkaline compound and a dispersant, mixing the admixture with first metal hydroxide, second metal hydroxide and an interlayer anion source and dispersing the mixture, and aging the dispersed mixture at a reaction temperature of 150˜250° C. under reaction pressure of 5˜30 kgf/cm2 for a period of time ranging from 20 min to 5 hours.
However, the method of preparing hydrotalcite disclosed in the above patents adopts a one-step synthesis method, and therefore it is difficult to maximize heat stability. Accordingly, there is a need for methods of preparing hydrotalcite able to maximize heat stability.